The present invention disclosed herein relates to an optical communication system, and more particularly, to a passive optical network (PON) system and an optical signal receiving method thereof.
Recently, Internet traffic is rapidly increasing due to continuous growth of video-based application services, which require real-time data transmission, and provision of telecommunication/broadcasting convergence services. In order to efficiently cope with the increase of traffic, network operators have continuously increased transmission bandwidths by utilizing Wavelength Division Multiplexing (WDM) technology in inter-metropolitan backbone networks and metro networks.
On the other hand, subscriber networks which distribute traffics transmitted from the backbone network to final subscribers have been used in a state that a typical Very-high-bit-rate Digital Subscriber Line (VDSL) and cable modem based transmission technology and a high-speed Ethernet based technology are mixed. Fundamentally, these technologies have a short network installation area and their transmission bandwidths are extremely limited in stably providing integrated services which are under intensive investigation. To solve these limitations, optical network technologies, which are under intensive development, aim to efficiently provide transmission bandwidth necessary for the integrated services to the final subscribers.
Optical network technologies, which are under intensive investigation, may be classified into a Time Division Multiplexing Passive Optical Network (TDM-PON) technology and a Wavelength Division Multiplexing Passive Optical Network (WDM-PON) technology. In the case of the TDM-PON, an optical line terminal (OLT) and a plurality of optical network units (ONUs) are connected together through a passive optical splitter, and a single transmission wavelength is shared at an optical layer by the plurality of ONUs. In the TDM-PON, a downstream data transmission is achieved by a Time Domain Multiplexing (TDM) scheme, and an upstream data transmission is achieved by a Time Division Multiple Access (TDMA) scheme based on a bandwidth reservation.
On the other hand, the WDM-PON constructs a logical point-to-point configuration by allocating individual transmission wavelengths to ONUs. Since data transmission between the OLT and the ONUs is achieved independently without any time division procedure, high transmission bandwidths can be provided to the subscribers. However, in the case of the WDM-PON, subscriber charge per bandwidth is high due to the expensive transmission system, and thus it is expected that somewhat long time is necessary to reach the practical utilization step. On the contrary, the TDM-PON is considered as next-generation optical network technology because it can efficiently use the same wavelength through the time division and its system price is relatively low. The TDM-PON technology may be classified into Ethernet-PON, G-PON, and B-PON according to the frame format of a transport layer, but their basic concepts of the upstream/downstream transmission control are equal to one another. In the case of the upstream transmission, since the data transmission from a plurality of ONUs and optical network terminals (ONTs) to the OLT, which is the common destination, is achieved by the shared link, an appropriate media access control (MAC) technology is required for preventing data collision. To this end, generally, the ONU and the ONT reserve bandwidth necessary to the next transmission period, based on a total amount of data accumulated in a buffer. After arbitrating such a reservation, the OLT allocates transmission time slots, that is, upstream bandwidths. Therefore, it is possible to maintain high network efficiency and also fairly allocate bandwidths to the ONUs. In this case, since data frames transmitted from the respective ONUs during the time slot period are the point-to-point communication where the primary destination is the OLT, the fairness of the bandwidth allocation can be easily maintained through the control of the time slots.
On the contrary, the downstream data transmission is achieved as follows. That is, all the data frames transmitted from the OLT are split at the optical layer by an optical splitter and broadcast to all the ONUs and ONTs, and the individual ONUs filter only the necessary frames from the received frames at the MAC layer, based on the destination address. In this case, if all the traffics are unicast frames, that is, frames directed to only the single destination, just like the case of the upstream transmission, the OLT can ensure the fairness of the bandwidth allocation by fairly allocating the downstream transmission time slots to the ONUs. However, in the case of the downstream transmission in the TDM-PON, a large amount of multicast traffics always exist due to VoD and SVD services or the like. These multicast traffics are simultaneously shared by a plurality of ONUs through the optical splitting.
FIG. 1 is a block diagram illustrating the architecture of a TDM-PON system. Referring to FIG. 1, a plurality of ONUs 30 through 60 are connected to one OLT 10. The OLT 10 and the ONUs 30 through 60 are connected together through an optical signal splitter 20. Each of the ONUs 30 through 60 shares optical lines with the OLT 10 and thus shares the installation cost of the optical lines and the cost of the OLT 10. The sharing of the optical lines can reduce the service charges of the ONUs. Therefore, as the number of the ONUs connected to one OLT 10 increases, the service charge per ONU is reduced. However, if a lot of ONUs are connected, optical loss occurs in the connection nodes. In addition, optical signals having a power higher than a specific level are required for detecting signals at an optical receiver. Thus, a light source having a high power is required for connecting more ONUs. If the number of the ONUs can increase even though the cost of the OLT shared by a plurality of ONUs increases, the cost reduction effect of the optical lines and the ONUs is greater than the increase in the cost of the OLT. Therefore, by increasing the power of the light source applied to the OLT, the service charge per unit ONU can be reduced. However, if the power of the light source applied to the ONU increases, the cost of the ONU increases and the service charge per ONU increases in proportion to the cost of the ONU. Increasing the output power of the ONU is economically inefficient. Accordingly, in the case of the upstream signal, there is a limitation in increasing the optical power. Furthermore, in order to compensate for optical loss occurring at the connection nodes of the ONUs, it is necessary to improve the receiver sensitivity of the optical receiver or compensate for the optical loss.
Therefore, there is an increasing demand for technologies that can reduce optical loss occurring at the optical distribution network and also flexibly cope with the increase of subscribers.